The New Hori­zons space­craft will ex­plore our so­lar sys­tem’s outer lim­its as it con­tin­ues its jour­ney of more than 3 bil­lion miles

It’s a jour­ney that has taken al­most a decade, and it’s go­ing to pass in the blink of an eye. But it could un­lock the se­crets of the ori­gin of the so­lar sys­tem.

On Tues­day, NASA’s New Hori­zons space­craft will zip past Pluto at more than 30,000 mph, hav­ing trav­eled more than 3 bil­lion miles since its launch in 2006. The flyby, which will col­lect reams of data, will be hu­mankind’s first en­counter with Pluto. And if you still con­sider Pluto a planet — which many do, even though in 2006 it was re­clas­si­fied as a “dwarf planet” — then this seems like the last fron­tier of ex­plo­ration in our cos­mic neigh­bor­hood.

But Pluto isn’t the last fron­tier. It’s hu­mankind’s first foray into the Kuiper Belt, a thrilling re­gion of space that starts just be­yond Nep­tune’s or­bit.

“The rea­son why we’re so ex­cited about ex­plor­ing Kuiper for the first time is that it gives us the best win­dow into what the so­lar sys­tem was like just af­ter form­ing 4.6 bil­lion years ago,” said Harold Weaver, New Hori­zons mis­sion sci­en­tist at Johns Hop­kins Univer­sity’s Ap­plied Physics Lab­o­ra­tory.

The re­gion, he said, has acted “al­most like a cos­mic re­frig­er­a­tor.” The ob­jects in­side the belt have been kept in­cred­i­bly cold since their for­ma­tion near the be­gin­ning of the so­lar sys­tem.

“They’re prob­a­bly more pris­tine sam­ples of the ma­te­rial that ex­isted at the time of the so­lar sys­tem’s for­ma­tion than any­thing in­te­rior of them,” Weaver said.

Sci­en­tists liken the en­counter to an as­tro­nom­i­cal ar­chae­o­log­i­cal dig: a look at the build­ing blocks on hand in the ear­li­est days. These build­ing blocks would come to­gether to cre­ate our own planet and to make it hab­it­able. And they would click into place to form the first life on Earth.

It is be­cause of the dis­cov­ery of the Kuiper Belt that Pluto is no longer a proper planet: The out­er­most re­gion of our so­lar sys­tem, its ex­is­tence was con­firmed in 1992, about 60 years af­ter Pluto— an ob­ject that re­sides in the in­ner­most re­gion of the belt — was dis­cov­ered. Sud­denly, Pluto wasn’t the outer limit, the tini­est, cold­est, most dis­tant planet. It was just one of per­haps hun­dreds of sim­i­lar ob­jects or­bit­ing at the fringes, sur­rounded by an es­ti­mated tril­lions of comets.

But get­ting into the Kuiper Belt isn’t just about ven­tur­ing to the outer lim­its of the so­lar sys­tem. The ob­jects in­side the Kuiper Belt are icy rem­nants of the for­ma­tion of the so­lar sys­tem, so they hold clues to its very begin­nings — and, by ex­ten­sion, to the ori­gin of life as we know it.

The belt has up­ended some of what sci­en­tists thought they knew about the so­lar sys­tem. To ex­plain the ex­is­tence of the new bod­ies, mod­els of the early so­lar sys­tem needed to be reeval­u­ated. Sci­en­tists now think, for ex­am­ple, that the plan­ets have moved sig­nif­i­cantly since their for­ma­tion: Jupiter might have been as close to the sun as Mars is, and Uranus and Nep­tune might have flipped po­si­tions. All those find­ings were mo­ti­vated by the need to ex­plain this brand new zone of the so­lar sys­tem.

Joel Parker, di­rec­tor of the South­west Re­search In­sti­tute’s space science and en­gi­neer­ing di­vi­sion, said that Pluto — while much sim­pler than our own planet and much more pris­tine — is very much alive.

“It’s a liv­ing planet. There are a lot of pro­cesses go­ing on, and in some ways Pluto has prob­a­bly changed a lot in the past 4.6 bil­lion years,” said Parker, whose team is in charge of at­mos­phere-mea­sur­ing in­stru­ments on the New Hori­zons mis­sion and the Euro­pean Space Agency's Rosetta mis­sion, which is or­bit­ing a comet mov­ing to­ward the sun. That comet, Parker said, is in some ways more pris­tine than Pluto — it came from the Kuiper Belt, as many comets do, and it changes only when it makes a rare pass close to the sun’s heat.

But when it comes to ex­am­in­ing those build­ing blocks, Pluto’s moons might be more help­ful than the dwarf planet it­self. Charon, Ker­beros, Nix, Styx, and Hy­dra prob­a­bly haven’t been very ac­tive in the past 4.6 bil­lion years. And un­like the comet Rosetta is study­ing, they’ve never left the cos­mic re­frig­er­a­tor they call home. Fig­ur­ing out their com­po­si­tions will help sci­en­tists un­der­stand how they formed and what the so­lar sys­tem was like back then.

Charon’s craters are also ex­pected to be help­ful: They shouldn’t have changed much since the early days of the so­lar sys­tem, so sci­en­tists can use them to cal­cu­late how many im­pacts the moon ex­pe­ri­enced back then — when some sus­pect the Kuiper Belt had 100 times as many ob­jects as it does now.

This ex­plo­ration need not end with Pluto and its moons. If NASA ex­tends the New Hori­zons mis­sion, its team can use the last of the probe’s fuel to di­rect it to­ward one of sev­eral other ob­jects they’ve de­tected in the Kuiper Belt.

So there is a lot more to the mis­sion than Pluto it­self, even if the dwarf planet is by far the most fa­mous char­ac­ter fea­tured.

“I have no doubt that when we look at Pluto up close and per­sonal, we’re go­ing to see things that we never could have seen be­fore from Earth,” Parker said. “There are go­ing to be things that make us have to re­vise our mod­els, and that’s why wed o this. It’s that next step in un­der­stand­ing the uni­verse.”

Parker is es­pe­cially ex­cited to find out more about Pluto’s re­la­tion­ship with Charon, its largest moon. The two form an odd pair: At 750 miles across, Charon is half the size of Pluto, mak­ing it the largest known moon rel­a­tive to its host planet. And in­stead of fol­low­ing the tra­di­tional courtship of amo on or­bit­ing around its host, Pluto and Charon or­bit in sync around a fixed point be­tween them.

They act more like a bi­nary plan­e­tary sys­tem, with Pluto’s smaller moons strug­gling to stay sta­ble around the pair’s strange cos­mic dance. But their com­po­si­tions seem to be rad­i­cally dif­fer­ent, and pin­ning down what they’re made of could help sci­en­tists un­der­stand how they formed — and why they act so strangely.

Pluto’s at­mos­phere is par­tic­u­larly in­ter­est­ing: Sci­en­tists al­ready know that it es­capes, seem­ingly evap­o­rat­ing into space. But it keeps com­ing back. Some­thing on the sur­face of the planet must re­plen­ish that at­mos­phere, but they aren’t sure what.

Christo­pher T. Rus­sell, prin­ci­pal in­ves­ti­ga­tor of NASA's Dawn mis­sion — which is or­bit­ing Ceres, another dwarf planet lo­cated be­tween Mars and Jupiter — pointed out that alien worlds can teach us a lot about things back home.

“On Earth, we have an en­vi­ron­ment that’s wrapped up in the cy­cle of wa­ter freez­ing and evap­o­rat­ing, rain and ice, and it’s all about the prop­er­ties of wa­ter,” Rus­sell said. But on Pluto, tem­per­a­tures as low as nearly mi­nus400 de­grees make all wa­ter solid. Mean­while, el­e­ments that are gases on Earth — such as car­bon diox­ide and ni­tro­gen — are able to form liq­uids or even solids at those tem­per­a­tures.

“If you look at wa­ter run­ning down a hill­side and com­pare it with, say, liq­uid ni­tro­gen run­ning down a hill­side, you might learn some­thing new about ero­sion for ex­am­ple, be­cause the prop­er­ties of the ma­te­rial are dif­fer­ent, so just the ba­sic process is the same,” Rus­sell said.

It’s clear that Pluto’s “de­mo­tion” hasn’t tar­nished it in sci­en­tists’ eyes. Dwarf planet or planet, it’s a strange new world ripe for ex­plo­ration.

“Hu­mans try to pi­geon­hole things and put them into cat­e­gories, but we have to re­al­ize that you can’t fit ob­jects into clearly de­fined cat­e­gories,” Weaver said. “Na­ture just doesn’t work that way, and Pluto is what it is in­de­pen­dent of how we try to la­bel it.”

And what it is, he said, is a body with at least five moons around it — a per­fect lit­tle sys­tem.

“My gosh, just think about the com­plex­ity of some­thing like that,” Weaver said. “How did it get to be like that? We just don't know.”

On Tues­day and Wed­nes­day, NASA is ex­pected to re­lease photos of Pluto that will turn the dwarf planet from a dis­tant cu­rios­ity into a real, com­plex, beau­ti­ful world.

“From a philo­soph­i­cal point of view, it’s the com­ple­tion of hu­man­ity’s first re­con­nais­sance of all the plan­ets in the orig­i­nal so­lar sys­tem, some­thing we started just 50 years ago,” Parker said. “To me, that’s as­tound­ing. Maybe in to­day’s fast-paced world that seems like a long time. But, you know. Space is big. Space is hard.”

TERRY RENNA/AS­SO­CI­ATED PRESS

The rocket car­ry­ing the NewHori­zons space­craft on a mis­sion to Pluto lifted off in Jan­uary 2006 from Cape Canaveral, Fla.

NASA/REUTERS

This im­age of Pluto— the most de­tailed yet— was trans­mit­ted from the space­craft’s Long Range Re­con­nais­sance Imager.